VOLCANOES

A volcano is a mound, hill or mountain constructed by solid
fragments, lava flows, and or dome-like extrusions deposited around a
vent from which the material is extruded. The vent is a conduit that
extends from the earth's upper mantle or lithosphere to the surface.
Most of the material is deposited close to the vent, but some is
carried high into the atmosphere to be spread by winds hundreds or
thousands of kilometers from the source.

Types of Volcanoes

The form, or shape, of a volcano is governed by the composition of
erupting magma and type of erupted products (volcaniclastic products
of various kinds such as pyroclastic and autoclastic
fragments; or effusive lava). Their shapes are determined in large part
by the explosivity of eruptions, and volume of water that
interacts with magma.

Shield Volcanoes

View northward toward Mauna Loa
volcano from Pohue Bay, south coast of Hawaii. The broad curving horizon line is the summit of Mauna Loa that
stands over 14,000 feet above sea level and 30,000 feet above the sea
floor. It is the highest mountain on earth.

Shield volcanoes are large volcanoes with broad summit
areas and low-sloping sides (shield shape) because the extruded
products are mainly low viscosity basaltic lava flows. A good example
of a shield volcano is the Island of
Hawaii (the "Big Island"). The Big Island is formed of five coalesced
volcanoes of successively younger ages, the older ones apparently extinct.
Mauna Loa, one of the main volcanoes, has a higher elevation than any
mountain on earth -- 9090 meters (30,000 feet) from the floor of the
ocean to its highest peak.

Shield volcanoes have summit calderas formed by
piston-like subsidence. Subsidence occurs when large volumes of lava
are emptied from underground conduits; withdrawal of support leads to
collapse. Many smaller pit craters also occur along fissure zones on
the flanks of the volcanoes. These form by collapse due to withdrawal
of magma along conduits.

Cinder cones (scoria cones)

Cinder cone at Little Lake, California.

Cinder cones are mounds of basaltic fragments.
Streaming gases carry liquid lava blobs into the
atmosphere that rain back to earth around the vent to form a cone. The lava blobs commonly solidify, or partially solidify, during
flight through the air before landing on the ground. They are called "bombs." If gas
pressure drops, the final stage cinder cone construction may be a lava flow
that breaks through the base of the cone. If abundant water
in the environment has access to the molten magma, their interaction may result in a maar volcano
rather than a cinder cone.

The longer the eruption
the higher the cone. Some are no higher than a few meters and others
rise to as high as 610 meters or more, such as Paricutin
volcano, Mexico, that was in nearly continuous eruption from 1943 to
1952. Along with pyroclastic activity were lava flows that
flowed from its base to destroy the village of Paricutin. Cinder
cones can occur alone or in small to large in groups,, or fields.

Composite Volcanoes ("Stratovolcanoes")

Composite volcanoes are built by multiple eruptions,
sometimes recurring over hundreds of thousands of years, sometimes
over a few hundred. Andesite magma, the most common but not the
only magma type, tends to form composite cones. Although
andesitic composite cones are built mostly of fragmental
debris, some of the magma intrudes fractures within the cones to form dike or sills.
In this way, multiple intrusive events build a structural
framework of dikes and sills that knits together the voluminous accumulation of
volcanic rubble. Such a structure can stand higher than cones composed only of
fragmental material. Composite cones can grow to such heights that their
slopes become unstable and susceptible to collapse from the pull of gravity.

Famous examples of composite cones are Mayon Volcano, Philippines,
Mount Fuji in Japan, and Mount Rainier, Washington, U.S.A. Some composite
volcanoes attain two to three thousand meters in height above their bases.
Most composite volcanoes occur in chains and are separated by several
tens of kilometers. There are numerous composite volcano chains on earth,
notably around the Pacific rim, known as the "Rim of Fire".

Lava domes form by the slow extrusion of highly viscous
silica-rich magma. Most domes are rather small, but can exceed 25 cubic km
in volume. Domal extrusions may end up as rather slow-moving lavas but many
begin explosively, forming reamed-out explosion pits blanketed by pyroclastic
debris. The explosive activity wanes as the gas content decreases. With
lowered gas pressures, the magma extrudes slowly as viscous lava that
forms thick stubby flows, or domes that are spinal or dome-shaped. As a
dome enlarges, its margins slowly creep outward as a lava flow with steep
cliff-like margins and a rubbly surface. If protrusion occurs on a steep
slope, dome margins can collapse in a dangerous mass of hot rubble
that can form pyroclastic flows.
Domes can be solitary volcanoes, form in clusters, grow in
craters or along the flanks of composite cones. A dome has been
growing slowly within the crater of
Mount St. Helens since the eruption of 1980. Domes have also filled
the crater of Mt. Pelée, Martinique, and many other volcanoes.

The eruption of Mount Unzen, Japan
taught volcanologists a
valuable lesson -- that the collapse of an active dome can cause pyroclastic flows to develop. From 1991 to 1995, the continued growth and partial collapse
of the Mount Unzen dome initiated hundreds of small but
highly destructive pyroclastic flows and surges.

Calderas

Photograph encompassing part of Crater Lake caldera, Oregon, U.S.A.
Diameter about 8 kilometers. View is toward the east and includes a
late-stage volcano in crater named Wizard Island.

Types and Origins of Calderas

Calderas are
circular to oblong depressions formed by collapse along arcuate fractures
associated with extrusion of pyroclastic materials. Their
diameters are many times larger than those of associated vents. They
may attain diameters up to 60 km across. The largest estimated
volume of erupted products is over 3500 cubic kilometers, and deposits
are known to have covered 25,000 square km. The frequency of such
voluminous eruptions is very low. Those with volumes of 500 cubic km
have a frequency of about 100,000 years. Such eruptions have occurred
at Long Valley, California (Mammoth area) with a caldera of 20 km
diameter; several have occurred in the Colorado Rocky Mountains (San
Juan Mountains), southern New Mexico, Los Alamos area, New Mexico
(Valles Caldera in the Jemez Mountains), and many other places.
Most calderas in western North America have developed on thick
continental crust. Intraoceanic calderas are commonly smaller in
size and eruptive volume and less silicic.
Another source of information about
calderas
is the U.S. Geological Survey.

The area of caldera collapse is about proportional to the
volume of erupted material. Depths of subsidence as indicated by
thickness of caldera fill is 1 to 3 km or more. Structural boundaries of
calderas are commonly single or composite ring fault zones along
which initial collapse took place. In deeply eroded calderas
these structural boundaries may be expressed by a ring dike
emplaced along arcuate faults during or after collapse.

Historical Perspective

Only since the 1950's and 60's
has it been generally accepted that there is a connection between silicic
volcanism and granitic plutonism. Many pyroclastic eruptions are
equivalent in volume to batholiths (crystallized magma of the
original magma chamber now exposed at the earth's surface), and it
has been demonstrated that many shallow granitic plutons
(intrusive igneous bodies) are roots of volcanoes.

The 1883 Krakatau eruption showed that caldera formation and
pyroclastic eruptions are interrelated. Pyroclastic flow
processes were first recognized following the 1902 eruption of Mont
Pelee, Martinique, and in 1942, a volcanologist from Berkeley,
Professor Howell Williams, demonstrated that many calderas form by
collapse following large pyroclastic eruptions. It is now
generally accepted that all silicic eruptions having volumes
greater than 50 km3 are associated with caldera collapse. The
many large Cenozoic ignimbrite sheets (pyroclastic flow and surge
deposits) in the western United States suggests that there are 250
to 500 caldera structures, although no more than 100 have been
identified; about one-fourth of them are associated with ore
mineral deposits.

Many calderas are associated with important ore
deposits, commonly lead-zinc-silver-gold, but also copper,
moybdenum, tungsten, beryllium and uranium, and are associated
with geothermal systems that originate in the volcano-plutonic
transition zone. Steam from geothermal systems is used to
run generators for electrical power.

Ignimbrite Fields and Calderas

There are many large
mineralized calderas and associated ignimbrite sheets of Cenozoic
age in the Western United States. An important example is the
San Juan volcanic field in the Rocky Mountainss of Colorado.

The San Juan
Volcanic Field is a 25,000 km2 erosional remnant of a composite
volcanic province that once extended over much of the southern
Rocky Mountains. Within the San Juan Field there are 17
large-volume ignimbrite sheets (greater than 100 km3) that have
been related to 17 caldera collapses.

Most of the San Juan Field is andesitic lavas and breccias
erupted between about 35 and 30 m.y. ago. The pyroclastic flow
eruptions occurred between about 30 and 23 m.y. ago. San Juan
calderas collapsed within clusters of slightly older andesitic
stratovolcanoes or within older calderas. Mineralization of some
caldera structures range in age from contemporaneous with caldera
collapse to as much as 15 m.y. younger.

The Caldera Cycle

Premonitory activity includes precaldera volcanism and
associated earth crustal movements that record the rise of a
shallow crustal batholith. Stratovolcanoes commonly occur as
clusters over the rising magma body, their positions being
controlled by linear or arcuate faults or by regional structures.

The second stage of the caldera cycle, culminating eruptions and caldera
collapse, is triggered by voluminous eruption of silicic
pyroclastic material. Initiating eruptions may be from central vents as
indicated by
thick fallout pumice beds that commonly precede the emplacement of
pyroclastic flow deposits. As eruption of ash removes support,
the roof above the shallow-source magma chamber collapses along
ring faults to produce the initial caldera depression. Culminating
eruptions occur along the ring faults. Collapse
probably occurs during the pyroclastic
eruptions. Unstable steep scarps outside the ring faults tend to
slump into the newly-formed depression, enlarging the topographic
diameter and depositing breccias formed by collapse within the
caldera. Thick pyroclastic flow deposits emplaced concurrently
with collapse commonly occupy most of the caldera crater.

Postcollapse Activity.

Postcollapse activity can include

(1) continued volcanism within or near the caldera,

(2) resurgent uplift of the caldera related to renewed rise
of the magma. Renewed rise of magma after caldera collapse has
caused uplift of many caldera areas. This is called resurgence.
It can consist of doming of the central floor of single calderas,
or be a broad regional uplift of one or more calderas and close-by
areas. Resurgent calderas are more numerous along the continental
margin arc of the Americas than in young volcanic arcs elsewhere
in the Pacific Basin. Resurgent structures are associated with
large calderas greater than 10 km diameter.

(3) sedimentation within the caldera basin commonly within a
lake, and

(4) hydrothermal activity and mineralization resulting from
the interaction of meteoric water and hot country rock.
Postcollapse volcanism can continue for millions of years.